1. Field of the Invention
The present invention relates to a control system for controlling an oscillatory-mirror light beam deflector with a three-mode or PID (proportional-plus-integral-plus-derivative) control loop.
2. Description of the Related Art
There has widely been used an image scanning recording and reproducing system for electrically processing image information borne by an original to produce a master in the fields of printing and photomechanical processes.
The image scanning recording and reproducing system basically comprises an image reading device and an image recording device. The image reading device reads the image information on the original in a main scanning direction while the original is being fed in an auxiliary scanning direction which is perpendicular to the main scanning direction, and converts the read image information into an electric signal. The image recording device processes the electric signal according to photomechanical processing conditions, converts the processed electric signal into an optical signal, and records the optical signal on a recording medium.
The image recording device has an oscillatory-mirror light beam deflector such as a galvanometer mirror, a resonant light beam deflector, or the like for deflecting a light beam at a high speed to record an image. The osciliatory-mirror light beam deflector has a solenoid or a motor coupled to a rotary driver and a reflecting mirror connected to the rotary driver. When an AC drive signal is supplied to the solenoid or the motor, the reflecting mirror oscillates to deflect a light beam applied thereto and scan a recording medium with the deflected light beam.
To record desired image information accurately on the recording medium, it is necessary to oscillate the reflecting mirror stably at all times. To meet such a requirement, the oscillatory-mirror light beam deflector is usually controlled through a feedback loop including a PID processor circuit.
In the PID processor circuit, a drive condition signal such as a deflection angle, which is produced by the oscillatory-mirror light beam deflector, is supplied to a differentiator, an integrator, and a proportional circuit to produce an adjustment signal. The adjusting signal and the drive condition signal are processed by a processing circuit to generate a new drive signal to be applied to the solenoid or the motor. The differentiator does not operate under steady conditions, but rather operates under transient conditions caused by the operation of an ON/OFF circuit. When the differentiator operates, the reflecting mirror, as it oscillates, may overshoot due to the characteristics of the multiplier used in the PID processor circuit or the individual attributes of the oscillatory-mirror light beam deflector employed. Overshooting may result in damage. If, however, the reflecting mirror oscillates with an insufficient amplitude, then the light beam deflected thereby will fail to fully scan the recording medium.